Why Not Razor Sharp?

So as a final (gee I hope it's final) postscript, I generally go the same route that Buck did when it created Edge 2000. I reprofile most blades to a lower angle and finish off with a sub 15 degree honed edge. When this isn't practical (I don't have time or I'm sharpening something with a rather hopeless profile) I like to leave a rougher finish on the edge. This gives me higher slicing effectiveness on a less acute edge. I worked that out sharpening bayonets. I figured this out in my first 10 years of blade sharpening and it has seemed to hold true for the 40 years that I have been sharpening since then.
 
Jeff Clark said:
Hi Frank, I get your point. If you look at that last diagram that I referred to the blade profile effects are most obvious as the cutting performance flattens out on the right side of the graphs. To compare apples to apples lets only look at the BG42 curves. As you pointed out the worn Edge2000 blade cuts better than the worn baseline edge. Some of that is due to the difference in honing angle, but much of it is also probably due to overall blade profile changes. The difference in performance in that region is about 1 on the depth of cut scale.

The place where the difference in honing angle does show up is on the left side of the graph. Note that the initial difference in performance is over 2 on the depth of cut scale. It looks like at least half of the initial higher cutting effectiveness of the Edge2000 is coming from its more acute edge angle. The more interesting thing is the difference in slope between the two curves. The curve for the BG42 edge honed at 20 degrees drops much more steeply than the curve for the BG42 edge honed at 13-16 degrees. That means that not only is the initial performance of the 20 degree edge lower than the performance of the 13-16 degree edge, it also dulls faster than the 13-16 degree edge. That was my point. Contrary to popular impression a more acute edge dulls slower than a less acute edge.
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But Jeff, that is where Mr. Stamp made his error. Mr. Stamp was kind enough to provide the reference from which he drew his data. It was a post here on BladeForums by CJ Buck in which he quoted an aritcle written by Buck Knives Engineering Superfisor, Bill Keys.
http://www.bladeforums.com/forums/showthread.php?t=127499

Unfortunately, the graphs are no longer showing in the post and the link to the data at Buck no longer works. But, the descriptions of what was tested are still there. Two BG42 blades were tested:
"BG-42 stainless steel with edge 2000 (E2K)"
"BG-42 without E2K- included angle of 40 degrees"

There was no comparison testing of only the edge angle being changed. It was an all or nothing change. One sample got the entire re-profile and the more acute edge, while the other got no reprofile and an obtuse edge.

So you cannot say how much effect was due to the edge angle, and how much was due to the rest of the profile change.

Side note: you cited the depth of cut data as being the most revealing. But that is where the overall blade profile would have the most effect.

Anyway, I do agree that edge angle makes a difference in cutting performance. I just think that Mr. Stamp stated conclusions that his data do not actually support. Not because the statistics were poorly done, but because he used data that did not properly represent his hypothesis.

me done, too.

All the best,
Frank R
 
Thar she be. Thanks, hardheart.
 
Because of the lack of consensus here about how terms are defined, it is easy to have misunderstandings.

Jeff Clark said:
Contrary to popular impression a more acute edge dulls slower than a less acute edge.
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It depends on how you define "dulls".
If you mean how fast it loses sharpness (edge diameter), this is not true. The more acute edge will lose it's high sharpness at a faster rate than the less acute edge (but it could stay sharper or cut better). If you said a more acute edge cuts easier for longer than a less acute edge, I'm with you. I think this is a difference in definitions.

Jeff Clark said:
The last plot on the web page illustrates that for ordinary wear (mildly abrasive wear) a thinly hollow ground 420HC blade can outperform a thicker grind on a BG42 alloy blade.
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I agree!

Jeff Clark said:
Take a look at the test results in the last chart in this paper. That is how knives "wear".
Click to expand...

That does not show "how knives wear" to me (especially after I posted about edge degradation compared to edge retention). I think about the mechanics of dulling when I think of "how knives wear". definitions again.
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Looking at Buck's testing, Buck was careful in making their conclusions, and I agree with the conclusions they made about the results.

What is not stated is the edge thickness at the edge bevel. Are they the same for all knives? I assume they are not. If so, what those tests might have shown as much as anything else is that a knife with the smaller edge thickness at the micro-bevel cuts better than a knife with a thicker edge at the micro-bevel. The other aspect of it does lend credence that a more acute edge has better edge retention than a less acute edge angle.

Blade thickness above the edge bevel can make a big difference cutting some materials, but shouldn't make a difference with CATRA testing because the wedge force of the blade does not come into play (but the wedge force of the edge bevel does). CATRA material is like a deck of cards, so each layer of material falls away as it is cut. I've heard this stuff is loaded with silica, kind of like cutting sandpaper. With other things being equal it would give the advantage to the most wear resistant steel. I would also be hesitant to make conclusions just based on the first 3 or 4 test cuts of the machine.
http://www.youtube.com/watch?v=WpIRLMdWsiE&feature=related

And if you did want to test whether a more acute edge has better edge retention than a higher angle edge, you should use blade and edge geometry so that the thickness of the blade where the edge bevel starts are equal. Testing 20 degree/side edge with a .03" bevel thickness against a 12 degree/side edge at a .015" bevel thickness is less than ideal, though it served Buck's purposes well.

Hopefully that made sense.
 
On my big field knives, 8"-11" a 40 or 50 degree inclusive edge with just a few steeper strokes on a 600 grit stone will make it through the weekend with just an occasional touch-up with a pocket stone. Shorter blades which I don't feel will primarily be choppers get laid back pretty thin and 10 or 12 strokes at the steeper angle for a finish keeps them going at barely shaving but effortless cash register receipt slicing. If I go to the strop loaded with the green polishing compound the only word that comes to mind is 'scary.' When I lend a knife out at work most guys are saying, "Yeah, yeah I know it's sharp" as I am telling them, 'Be careful with that, it's real sharp.'
 
I stand by my last explanation of how to pull information out of the graphs. The steeper slope at the beginning of the graph for the older blade profile indicates faster loss of sharpness for 20 degree edge. The overall blade profile is not changing, that is the edge wearing down. Slopes on graphs are a measurement of rate of change of something. In this case it is the rate of change of the edge performance because that is all that would change that fast.
 
I stand by your standing by your last statement, too, Jeff. Some mean-spirited dude who got bonked on the head by an apple (I hear he got banned from Rosicrucian Forums) observed a law of motion that can be looked at as saying cuts that take more force to make a cut also put more force against the edge.
 
Hi Vivi, do you have pictures of this edges?

I put a 26 deg angle on my sebenza, he edge is about 3 mm. Yours should be 5 mm or more!

Marthijn
 
I Think that a knife can be to sharp. For me is it when I start bleeding without noticing the cut!

Marthijn
 
holst035 said:
Hi Vivi, do you have pictures of this edges?

I put a 26 deg angle on my sebenza, he edge is about 3 mm. Yours should be 5 mm or more!

Marthijn
Click to expand...

Here are two older photos of the UKPK.




Currently the back bevel is difficult to distinguish from the rest of the grind, because it goes about half way up the blade and convexes at the top. Looks like a zero grind except for the microbevel.
 
Jeff Clark said:
I stand by my last explanation of how to pull information out of the graphs. The steeper slope at the beginning of the graph for the older blade profile indicates faster loss of sharpness for 20 degree edge. The overall blade profile is not changing, that is the edge wearing down. Slopes on graphs are a measurement of rate of change of something. In this case it is the rate of change of the edge performance because that is all that would change that fast.
Click to expand...

But what is changing is the contact area of the edge to the stock material. With a more obtuse angle, a slight amount of wear will create a greater increase in force over a more gradual change in thickness/lower grind angle. I would guess that is why the change in slope is more abrupt than the one for the lower angles. There is a marked drop off in sharpness as a greater surface area contacts the test material from the initial wear, and then this wide contact area wears more slowly after, getting a very flat curve.

A thicker piece of metal naturally wouldn't wear down faster than a thinner one, but the depth of cut will fall off dramatically with little wear when dealing with an edge thickness that increases at a greater rate.

I think a lot of people think dulling is just wearing away of metal, with no thought of to what happens when the edge does wear. Which is why they automatically equate thinner edges to lower edge life.
 
We all know that the apex of a blade edge has a dramatic influence on how most materials are cut. We take it for granted, but it is pretty remarkable that stress concentrates approximately at the point where a material contacts that apex and the material cleaves there. A tool like a Sharpmaker does not reshape much of a blade and yet by refining just the apex of blade edges it is a product that garners much praise by a multitude of users. Conversely when the apex of an edge is degraded it reduces cutting performance to a surprising degree for such a small loss of blade material (or reshaping of blade material if it just rolled over). This is sort of where I have always focused my understanding of what we call "dulling".

I always visualized the furthest projection of the blade edge as a having a triangular cross-section. This section is only a millimeter or so high and is subject to wearing primarily from the apex down. As it wears the apex becomes rounded rather than pointed. I haven't looked at blades all that much under magnification, but when a blade gets dull I can generally see a reflection from where the apex of the edge used to be. Unless I have been rubbing the blade on something very hard I expect the apex to be closer to rounded than flat on the apex. As this happens the stress concentration that the acute apex creates where it presses against material starts spreading out and being less effective at overcoming the cohesion of the material you are attacking. At any given point of wear I assume that the effective diameter of this rounded edge cross-section is roughly the width of the remaining sides of the worn triangle. With a more acute honing angle this width will be smaller for a given amount of material removed from the edge.

Think of it in terms of areas of cross sections of edge. You could have two triangular cross sections with equal areas, but different base widths. If you assumed that equal blade use will remove equal amounts of material from the edge (I think this is pessimistic) you would remove equal volumes from the edge or equal areas if you just look at the cross section. Look at the sketch below. These two triangles have equal area. If you start wearing them down from right to left which one is wider when you get to the left side? At equal rates of wear (measured by volume of removed material) you should reach the left side of both of them at the same time. You are left with a wider edge (probably best described as a wider radius of curvature) with the less acute blade profile. Just looking at the geometry has always made me expect less acute edges to dull faster.
 

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